Diffusion transfer process utilizing 2-mercapto imidazoles

ABSTRACT

2-MERCAPTO IMIDAZOLES ARE UTILLIZED IN DIFFUSION TRANSFER PROCESSES TO GIVE POSITIVE TRANSFER PRINTS OF IMPROVED QUALITY OVER A BROAD RANGE OF PROCESSSING TEMPERATURES.

United States Patent Office Patented Oct. 17, 1972 3,698,900 DIFFUSION TRANSFER PROCESS UTILIZING Z-MERCAPTO IMIDAZOLES Edward J. Johnson, .ln, Tewkshury, Mass., assignor to Polaroid Corporation, Cambridge, Mass.

No Drawing. Continuation of abandoned application Ser. No. 698,719, .Ian. 18, 1968. This application Aug. 23, 1971, Ser. No. 174,123

Int. Cl. G03c 5/54 US. CI. 96-29 4 Claims ABSTRACT OF THE DISCLOSURE Z-mercapto imidazoles are utilized in diffusion transfer processes to give positive transfer prints of improved quality over a broad range of processing temperatures.

BACKGROUND OF THE INVENTION This application is a streamlined continuation of application Ser. No. 698,719 filed Jan. 18, 1968 now abandoned.

This invention relates to photography and, more particularly, relates to diffusion transfer processes wherein an exposed silver halide emulsion is developed and an imagewise distribution of unexposed silver halide is formed and transferred, at least in part, to an imagereceiving layer.

In silver diffusion transfer processes, a latent image in a photo-exposed halide emulsion is developed with a silver halide developing agent in the presence of a silver halide solvent. Almost simultaneously with the development of the latent image, the silver halide solvent reacts with the silver halide in the unexposed and undeveloped areas of the emulsion to form a soluble, difiusible silver complex. The soluble silver complex is, at least in part, transferred to an image-receiving stratum where the silver thereof is precipitated to form a positive print.

It is desirable that such processes operate to give prints of good quality over a wide temperature range. In a diffusion transfer process at elevated temperatures, the reactions of solution, transfer and development which are balanced at 70 F. are accelerated non-uniformly. The photographic result of such behavior is a print of low contrast with grey background instead of rich blacks. The highlights will be dull and grey with no crispness.

It has been proposed to counteract these effects by employing certain compounds used in conventional tray or wet developmen photography to counteract the tendency of a silver halide emulsion to fog, i.e., to become developable without photoexposure, when developed at temperatures above room temperature. (The term antifoggants has frequently been used as a class designation for compounds found to be useful in counteracting such fogging tendencies.) These compounds frequently are not suitable for use in diffusion transfer processes and, if effective under high temperature-rapid development conditions in these processes, they tend to retard the formation of a silver transfer image to such an extent that a substantial reduction in effective film speed (exposure index) results.

SUMMARY OF THE INVENTION It is therefore the primary object of the present invention to provide diffusion transfer processes and processing compositions useful therein that produce, without any decrease in effective film speed, positive prints having improved contrast and whiter highlights at processing temperatures ranging up to and in excess of 100 F.

Other objects of the present invention will in part be obvious and will in part appear hereinafter.

It has now been found according to the present invention that the above and other objects may be accomplished if the diffusion transfer process is carried out in the presence of Z-mercapto imidazole(s). Though certain of these imidazoles have found utility as toning agents in diffusion transfer processes to provide black rather than brownish or sepia prints, it is quite unexpected that such compounds will materially reduce or substantially eliminate greying of the highlights in the positive and at the same time actually provide an increase in effective film speed.

The invention accordingly comprises the processes involving the several steps and the relation and order of one or more of such steps with respect to each of the others, and to compositions possessing the features and properties which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is primarily concerned with the processing, in the presence of Z-mercapto imidazole(s), of an exposed silver halide emulsion to give directly and, without further exposure, a positive print obtained by the development of the emulsion and by the transfer of at least a portion of the silver of the unexposed regions of said emulsion to a suitable image-receiving element Where the silver is precipitated to produce a visible image.

In one form of the process, the development of the latent image and the transfer and precipitation of the silver from the undeveloped areas of the silver halide emulsion occur almost simultaneously. A liquid processing composition is so applied to the surface of a photosensitive silver halide element as to be absorbed in part into the emulsion of said element and this single liquid application develops the exposed silver halide and causes the formation of a soluble silver complex with the remaining undeveloped silver halide. The processing composition is best applied in a thin, uniform, relatively viscous layer, for example, by being spread on the emulsion. In a preferred form of the process, the emulsion and image-receiving layers are in superposed relation, and the liquid processing composition is applied, e.g., as by being spread between the photosensitive and imagereceiving layers as described in detail in US. Pat. No. 2,647,056 issued July 28, 1953 to Edwin H. Land and numerous other patents.

The class of mercapto-substituted imidazoles particularly useful for practicing the present invention have at least one tautomeric form that may be represented by the following structural formula:

I SM

wherein R and R each are usually selected from the group consisting of hydrogen, alkyl, phenyl and carboxy (including the ammonium and alkali metal salts thereof) and M represents hydrogen, ammonium or an alkali metal. By alkali metal is meant a metal in Group I-A of Mendeleevs Periodic Table as exemplified by sodium and potassium. It will be understood that the R components are selected such that the compound will be soluble in the particular processing composition used.

Typical of the Z-mercapto imidazoles that may be employed in the present invention are:

In carrying out the present process, a single 2-mercapto imidazole may be employed, or a mixture of two or more such compounds may be employed. For achieving the benefits of the present invention most efiiciently, the imidazole(s) is preferably incorporated in the processing composition.

The Z-mercapto imidazoles may be used advantageously over a relatively wide range of concentrations to give transfer prints of satisfactory pictorial quality at the various processing temperatures. The minimum concentration should be that amount sufficient to decrease D in the transfer print. The optimum concentration will depend upon the particular emulsion, the processing composition, etc., and can readily be determined in each case. When the 2-mercapto imidazole is incorporated into the liquid processing composition, it is ordinarily added in amounts ranging between about 0.5 and 4 grams per liter of composition.

As indicated above, the liquid processing composition contains certain ingredients, some of which may be added wholly or in part to the composition during the spreading thereof as by being dissolved into the composition from the photosensitive and/or image-receiving elements, but which are most conveniently introduced therein prior to spreading. The essential ingredients are (a) a silver halide developing agent(s), i.e., a substance capable of reducing the exposed silver halide of the latent image to silver,

(b) a silver halide solvent(s), i.e., a substance capable of forming with the unexposed silver halide a silver complex which is soluble in the particular liquid vehicle used for the processing composition and (c) a Z-mercapto imidazole(s) as defined above for increasing the effectiveness of the process for producing positive transfer prints of good pictorial quality at processing temperatures up to and in excess of F.

In the processing composition, any suitable developing agent(s) and silver halide solvent(s) may be employed. Among the developing agents that may be employed are benzene and naphthalene compounds having hydroxyl and amino substituents ortho or para to one another, e.g., hydroquinone, chlorohydroquinone, bromohydroquinone, toluhydroquinone, t-butyl hydroquinone, p-methyl aminophenol sulfate, triaminophenol and pyrogallol.

The silver halide solvent may be selected from any of those known to the art. Particularly useful silver halide solvents include the alkali thiosulfates, e.g., sodium thiosulfate and cyclic amides, e.g., uracil.

Preferably, the processing composition also contains a thickening agent to increase and impart the desired viscosity characteristics. A viscosity of from 1,000 to 200,000 centipoises at a temperature of 20 C. has been found satisfactory for permitting the composition to be readily controlled during and after spreading. Illustrative of suitable thickening agents are carbohydrates, e.g., starch; gums, e.g., gum arabic; and plastic materials, e.g., hydroxyethyl cellulose, sodium carboxymethyl cellulose and the sodium salts of polyacrylic and polymethacrylic acids.

Additionally, the composition contains an alkali, e.g., sodium hydroxide and may contain a restrainer, e.g., potassium bromide; a preservative, e.g., sodium sulphite; or other adjuvants as conventionally used in diffusion transfer processes. The developing agent, silver halide solvent and any ad-juvant employed, such as a thickener, are used in conventional amounts to achieve the desired effects.

As mentioned hereinabove, in a preferred embodiment of the present invention the processing composition is spread in a uniformly thin layer between the superposed surfaces of a photoexposed silver halide emulsion and an image-receiving element, which procedure may be accomplished, for instance, by advancing the elements between a pair of pressure-applying rollers. In such a process, the liquid processing composition, including the developing agent, the silver halide solvent and the 2-mercapto imidazole may be conveniently provided in a rupturable container attached to either the photosensitive element and/or image-receiving element such that when the elements are superposed the container is so positioned as to be capable upon rupture of releasing its contents in a substantially uniform layer between and in contact with the surfaces of each of the elements.

Such containers are preferably inexpensive and disposable, and so constructed as to be capable of retaining the processing composition therein for relatively long periods of time without vapor loss or oxidation. Examples of containers that may be used for this purpose are described in US. Pat. No. 2,634,886 issued to Edwin H. Land. Generally, the containers are fabricated from a blank comprising a 3-ply sheet material including respectively an outer lamina of a strong deformable sheet material, e.g., kraft paper; an intermediate lamina impervious to the vapor of the processing composition, e.g., a metal foil; and an inner lamina inert and impervious to the processing composition, e.g., a thermoplastic resin layer. The container blank is folded upon itself and sealed in such a manner as to provide a container having a fluid-containing capacity and a sealed marginal edge which may be substantially unsealed throughout a predetermined portion of its length upon application of stress to the container.

The photosensitive element may comprise any of the commercially available silver halide emulsions such as gelatino silver chloride, chlorobromide, chloroiodide, chlorobromoiodide and bromoiodide emulsions which may be coated on any suitable support, for example, glass, paper, and plastic film base. The present compositions and processes, however, are particularly useful for improving the results obtained with the higher speed photosensitive emulsions.

The emulsions may be chemically sensitized with sulphur compounds such as thiourea; with reducing substances such as stannous chloride; with noble metals usch as gold or platinum; with amines and with quaternary ammonium compounds. Also, the emulsion may contain accelerators, coating aids and other such addenda where desired.

The image-receiving element may be any of those conventionally used in diifusion transfer processes and comprises a suitable support, such as baryta paper, which may have an outer layer formed of a natural or synthetic resin or a mixture thereof, which layer comes into contact with the processing composition. The image-receiving element preferably contains silver precipitating agents or nuclei since the presence of such materials during the diffusion transfer process has a desirable effect on the amount and nature of the silver precipitated in the formation of the positive print. Examples of materials suitable for this purpose are the metallic sulphides and selenides, thiooxalates, thioacetamides and colloidal metals disclosed in U.S. Pat. No. 2,698,237 issued Dec. 28, 1954 to Edwin H. Land. Also, as disclosed in this patent, it is desirable to provide on the support a continuous film consisting of submacroscopic agglomerates of minute siliceous particles as a vehicle for the silver precipitating agents or nuclei to enhance the aggregation of silver into its most eifective form.

To illustrate the efficacy of the subject imidazoles in composition in an amount of 0.005 gram per 10 cc. (cubic centimeters) of composition.

A series of the above-described photosensitive elements were exposed to the same subject matter under identical conditions and then advanced in superposed relationship with the aforementioned image-receiving elements between a pair of pressure-applying rollers to spread the liquid processing composition between the elements in a layer about 1.8 mils thick. After an imbibition period of 60 seconds in the cold and 10 seconds at both room and elevated temperatures, the photosensitive and image-receiving elements were separated to uncover the positive transfer prints.

The exposure ratings and density characteristics of the prints produced are given in the following Table I. The control consisted of the above processing composition with no imidazole present.

Exposure rating is given in terms of Diffusion Transfer Exposure Index. This term as used herein refers to the exposure index to which an A.S.A. calibrated exposure meter should be set to determine the proper exposure to which a negative for use in a silver diffusion transfer process must be subjected in order to obtain a satisfactory positive print and may be based on a curve relating original exposure of the negative to the density in the resultant positive. conventionally, the Diffusion Transfer Exposure Index of a silver halide transfer process is determined by plotting a characteristic curve of the reflection density of the positive as a function of the log exposure of the negative, determining the exposure in meter-candle-seconds at the point on this curve corresponding to a density of 0.50, and dividing the constant, 4.0, by the exposure so determined.

TABLE I Difiusion transfer exposure index Maximum density Minimum density Cold Room Hot Cold Room Hot Cold Room Hot Control 960 264 368 1. 50 1.61 1. 0. 01 0. 23 0. 43 Example... 1, 420 1, 920 940 1. 58 1. 62 l. 43 0. 00 0. 01 0. 05

Grams Hydroxyethyl cellulose 43.7 Sodium sulfite 31.1 Sodium hydroxide 50.7 Sodium thiosulfate pentahydrate 88.5 Triarninophenol dihydrochloride 5.9 t-Butyl hydroquinone 16.8

Water, 1,000 cc.

In formulating the above, the hydroxyethyl cellulose was dissolved in water with stirring at room temperature. Thereafter, the remaining ingredients except for the devel- In another test, Z-mercapto imidazole was added in an amount of 0.00015 gram per 10 cc. to an alkaline processing composition which comprised the following:

Using this composition, a series of prints were prepared at the three different temperatures given above using the same procedure except that the imbibition time was 45 seconds at room temperature and in the hot and 120 seconds in the cold. The photosensitive element used comprised a gelatino silver iodobromide emulsion on a paper base, and the image-receiving element comprised a cellulose diacetate layer containing a nickel sulfide precipitant coated on a paper base and hydrolyzed to a depth of 0.00005 inch.

The exposure index and maximum and minimum densities for the prints obtained are given in Table H wherein the control represents prints prepared with the processing oping agents were stirred into the solution and finally, the composition without mercapto-imidazole present.

TABLE II Ditfusion transfer exposure index Maximum density Minimum density Sample Cold Room Hot Cold Room Hot Cold Room Hot Control 1, 300 800 675 1. 58 1. 64 1. 58 0. 00 0. 07 0. 10 2-rnercapto imidazole 1,800 1,780 1,820 1.60 1.63 1 59 0.00 0.00 0.02

triaminophenol and hydroquinone were dissolved therein.

It will be noted from the data set forth in the tables The Z-mercapto imidazole was then incorporated into the 5 that D of the positive at room and elevated temperatures is materially reduced, Without materially affecting D by processing in the presence of the 2-mercapto imidazole. It will be noted further that the speeds in terms of Diffusion Transfer Exposure Indices obtained in the presence of this compound are considerably higher at the various processing temperatures as compared to the exposure indices of the control prints. This improvement in minimum density characteristics, together with increased speeds over the entire temperature range ensures the production of high quality transfer prints at temperatures slightly above freezing up to and in excess of 100 F.

It will be apparent that it is within the scope of the present invention to modify the processing composition described above by altering the relative proportions of the ingredients and/or by the substitution of developing agents, silver halide solvents, alkalis and so forth.

Since certain changes may be made in the above processes and compositions without departing from the scope of the present invention, it is intended that all matter contained in the above description be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a difiusion transfer process which comprises the steps of developing exposed silver halide of a photosensitive silver halide emulsion with a silver halide developing agent and reacting unexposed silver halide of said photosensitive emulsion with a silver halide solvent to form with said unexposed silver halide an imagewise distribution of a soluble silver complex, transferring at least a portion of said imagewise distribution of said silver complex by diffusion in alkaline solution to an imagereceiving element and reducing said transferred silver complex to provide a silver image, the improvement which comprises conducting said process in the presence of at least one compound selected from those represented by the formula HN yN C wherein R and R each are selected from the group consisting of hydrogen, alkyl, phenyl and carboxy and M is selected from the group consisting of hydrogen, ammonium and an alkali metal, whereby the minimum density of the transfer image is decreased, said compound being soluble in said alkaline solution.

2. A process according to claim 1 wherein said compound is incorporated in said solution.

3. A process according to claim 2 wherein said compound is present at a concentration of between about 0.5 and 4 grams per liter of solution.

4. A process according to claim 1 wherein said compound is Z-mercapto imidazole.

References Cited UNITED STATES PATENTS 2,647,056 7/1953 Land -88 3,017,270 1/1962 Tregillus et a1. 9658 X 3,335,009 8/1967 Rasch et al. 9661 3,353,957 11/1967 Blake 9629 3,368,899 2/1968 Blake 96107 NORMAN G. TORCHIN, Primary Examiner W. H. LOUIE, JR., Assistant Examiner US. Cl. X.R. 9658, 109, 66.5 

